of Saturated Vapours of Different Substances. 725 



is consistent with a general relation connecting vapour- 

 pressnre and temperature. This form only will be referred 

 to here, and it may be stated thus : — 



If T and T be the temperatures at which two substances 

 have the same vapour-pressure, and T', T ; be two tempe- 

 ratures at which the same substances have another value of 

 pressure the same for both, then 



T T ' 



~ — ~ =c(T — To'), where c is a constant. . (1) 



,'T 

 Hence ifr'^ be plotted against T a straight line will be 



obtained. 



In the Phil. Mag. for September 1902 (p. 337) the late 



Prof. J. D. Everett subjects this law to the criticism that it 



is equivalent to neglecting the term F| v 2 in Rankine's vapour- 

 pressure equation 



This is shown by writing equation (1) in a form equivalent to 



^_ — hi — 1 

 T To ' 



and then pointing out that Rankine's shortened formula leads 

 to this equation : Hfor the shortened formula gives 



, 13 /3 



logp=a— f£=* -yi 



whence fi ___ /3 _ 



T To- a - a o- 



It would seem, however, that the criticism that the law 

 will hold only when squares of the reciprocal temperatures 

 can be neglected, is not justifiable ; for it rests on the 

 assumption that we are restricted to Rankine's form of 

 equation. 



In order that Ramsay and Young's law shall be accurately 

 true, all that is necessary is that 



V % A B 



f(p)=A- T 



where the left-hand side may be any function of the pressure 

 which is identical for the substances in question. 



